1. Field of the Invention
The invention relates generally to closed loop power control systems, and more particularly to closed loop power control systems for use in wireless applications such as cell phone technology.
2. Related Art
Closed loop feed back power control loops are commonly used in a number of contemporary applications, including, for example, wireless communication systems. In such systems, a power amplifier amplifies a modulated signal to be transmitted at a desired level such that the radiated power of the transmitted signal from the system""s antenna is of sufficient strength to fall within the dynamic range of the intended receiver.
In many applications, closed loop systems are used to control radiated power to within a relatively narrow range. For example, the GSM transmission protocol requires that transmitted RF power be regulated within +/xe2x88x922 dB of a nominal value. Closed loop systems typically accomplish this goal through the use of a feed back loop. Such feed back systems can include, for example, a power detector associated with a coupler at or near the antenna to detect the actual transmitted power level.
Power detection/monitoring may be accomplished with a logarithmic detector system. The detected power level can be compared with a desired power level to determine a difference between actual and desired power levels. This difference signal is processed in some manner and used to generate a feed back signal to the power amplifier to control the output level.
One drawback of contemporary closed-loop systems is that the power amplifier typically has a characteristic or response curve having highly non-linear regions (e.g., in its saturation regions). As such, the response of the overall system, even with the feed back loop, is non-linear. This can result in less-than-desirable response characteristics of the entire system. For example, if the loop is operating in a region where there is too little gain, loop response may be slower than desired. If on the other hand, the loop is operating in a region where there is too much gain, the system may go into oscillation.
In a GSM system, slow loop response due to non-linear responsiveness of the power amplifier can be quite troublesome because, to meet protocol requirements, the transmitted signal power must be able to go from start-up power to full power (e.g., from near 0 watts to 2 watts) in less than 28 microseconds.
Accordingly, an object of the subject invention is a power control closed-loop system with improved linearity in relation to the prior art.
Another object is a power control closed-loop system which overcomes the disadvantages of the prior art.
Further objects of the subject invention include utilization or achievement of the foregoing objects, alone or in combination.
Additional objects and advantages are set forth in the description which follows or will be apparent to those of ordinary skill in the art.
The present invention provides a system and method for improving linearity in a closed loop power control system. According to one embodiment of the invention, a feed back loop including a logarithmic detector and a bipolar power amplifier is provided. The power amplifier is driven by a driver circuit which compensates for non-linearities in the system.
In one application, a self-adapting driver circuit is used to correct for non-linearities in a closed loop power amplifier system.
Specifically, in one embodiment, a supplemental current supply circuit is connected in series between the comparator and power amplifier of a closed loop power amplifier system. The supplemental current supply circuit senses when power amplifier is saturating and supplies extra current to roughly linearize the gain of the system.
According to one aspect of the invention, the closed loop power control amplifier system can be implemented in a wireless communication system such as, for example, GSM. Specifically, a closed loop power control amplifier system is provided comprising: a power amplifier configured to amplify a signal for transmission by an antenna over a communications channel; a power level sensor configured to sense the power level supplied to the antenna and generate a first signal in response thereto; a comparator configured to compare the first signal and a desired output power level and to provide a difference signal indicating the difference between the two; a power amplifier driver circuit for driving the power amplifier by a driving signal in accordance with the difference signal, wherein the power amplifier driver circuit senses when the power amplifier operates in a non-linear region and boosts the driving signal to correct for the non-linearity of the power amplifier.
Further features and advantages of the invention as well as the structure and operation of various embodiments of the invention are described in detail below with reference to the accompanying drawings.